|IUPAC name||phosphorus tribromide|
|Other names||phosphorus(III) bromide|
|ECHA InfoCard||Lua error in Module:Wikidata at line 879: attempt to index field 'wikibase' (a nil value). Lua error in Module:Wikidata at line 879: attempt to index field 'wikibase' (a nil value).|
|Molar mass||270.70 g/mol|
|Appearance||clear, colourless liquid|
|Density||2.852 g/cm3, liquid|
|Molecular shape||trigonal pyramidal|
|Main hazards||corrosive, toxic, reactive|
with water and alcohols
|R-phrases||14, 34, 37|
|Except where noted otherwise, data are given for|
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references
Please Take Over This Page and Apply to be Editor-In-Chief for this topic: There can be one or more than one Editor-In-Chief. You may also apply to be an Associate Editor-In-Chief of one of the subtopics below. Please mail us  to indicate your interest in serving either as an Editor-In-Chief of the entire topic or as an Associate Editor-In-Chief for a subtopic. Please be sure to attach your CV and or biographical sketch.
Phosphorus tribromide is a colourless liquid with the formula PBr3. It fumes in air due to hydrolysis and has a penetrating odour. It is widely used in the laboratory for the conversion of alcohols to alkyl bromides.
Phosphorus tribromide, like PCl3 and PF3, has both properties of a Lewis base and a Lewis acid. For example, with a Lewis acid such as boron tribromide it forms stable 1:1 adducts such as Br3B-PBr3. At the same time PBr3 can react as an electrophile or Lewis acid in many of its reactions, for example with amines.
The mechanism (shown for a primary alcohol) involves initial activation of the alcohol oxygen by the electrophilic phosphorus (to form a good leaving group), followed by an SN2 substitution at the alcohol carbon.
Because of the SN2 substitution step, the reaction generally works well for primary and secondary alcohols, but fails for tertiary alcohols. If the reacting carbon centre is chiral, the reaction usually occurs with inversion of configuration at the alcohol carbon, as is usual with an SN2 reaction.
The main use for phosphorus tribromide is for conversion of primary or secondary alcohols to alkyl bromides, as described above. PBr3 usually gives higher yields than hydrobromic acid, and it avoids problems of carbocation rearrangement- for example even neopentyl bromide can be made from the alcohol in 60% yield.
Another use for PBr3 is as a catalyst for the α-bromination of carboxylic acids. Although acyl bromides are rarely made in comparison with acyl chlorides, they are used as intermediates in Hell-Volhard-Zelinsky halogenation]. Initially PBr3 reacts with the carboxylic acid to form the acyl bromide, which is more reactive towards bromination. The overall process can be represented as
On a commercial scale, phosphorus tribromide is used in the manufacture of pharmaceuticals such as alprazolam, methohexital and fenoprofen. It is also a potent fire suppression agent marketed under the name PhostrEx.
PBr3 evolves corrosive HBr, is toxic, and reacts violently with water and alcohols.
In reactions that produce phosphorous acid as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to give phosphine which can cause explosions in contact with air.
- ^ N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
- ^ Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
- ^ J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
- ^ The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
- ^ R. R. Holmes, Journal of Inorganic and Nuclear Chemistry 12, 266-275 (1960).
- ^ L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
- ^ George C. Harrison, H. Diehl, in Organic Syntheses Collective Volume 3, p 370, Wiley, New York, 1955.
- ^ L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 1051, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.